Piston ring

ABSTRACT

A piston ring which is provided with a carbon-based coating which has a low friction property and wear resistance, that is, a piston ring which has a carbon-based coating formed over its sliding surface, in which piston ring, the coating is a multilayer coating comprised of two types of layers having different hardnesses laminated in at least two layers, a hardness difference between the two types of layers is 500 to 1700 HV, a high hardness layer has the same or greater thickness than a low hardness layer, and the coating as a whole has a thickness of 5.0 μm or more. The high hardness layer has a thickness of 5 to 90 nm. The surface on which the coating is formed has a base material roughness of 1.0 μmRz or less.

This application claims benefit of Serial No. 2011-069676, filed 28 Mar. 2011 in Japan and which application is incorporated herein by reference. To the extent appropriate, a claim of priority is made to the above disclosed application.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to a piston ring for an internal combustion engine.

2. Description of the Related Art

A carbon-based coating is known as a coating which has a low coefficient of friction, but the coating stress is high and self fracture occurs, so increasing the thickness is difficult. Japanese Patent Publication (A) No. 2006-008853 proposes a carbon-based coating with a low coefficient of friction which is made a multilayer structure with a surface-most layer comprised of a soft layer and with soft layers and hard layers alternately laminated and which enables a smooth surface to be easily obtained.

See the above Japanese Patent Publication (A) No. 2006-008853 for the related art.

Japanese Patent Publication (A) No. 2006-008853 does not define the hardness difference between the hard layer and the soft layer. However, in Japanese Patent Publication (A) No. 2006-008853, the soft layer is meant as a fitting layer, while the hard layer is meant as a wear resistance layer, so the hardnesses are, in Example 1, soft layer: 18 GPa and hard layer: 42 GPa and, in Example 2, soft layer: 20 GPa and hard layer: 41 GPa. The hardness difference between the soft layer and hard layer was large.

SUMMARY OF THE INVENTION

An object of the present invention is to provide a piston ring which is provided with a carbon-based coating which has a low friction property and excellent wear resistance.

The present invention provides a piston ring which has a carbon-based coating formed over its sliding surface, in which piston ring, the coating is a multilayer coating comprised of two types of layers having different hardnesses laminated in at least two layers each, a hardness difference between the two types of layers is 500 to 1700 HV, a high hardness layer has the same or greater thickness than a low hardness layer, and the coating as a whole has a thickness of 5.0 μm or more.

Preferably, the high hardness layer has a thickness of 5 to 90 nm.

Preferably, the surface on which the coating is formed has a base material roughness of 1.0 μmRz or less.

Preferably, a surface-most layer of the coating is a high hardness layer.

Preferably, a low hardness layer of the coating contains carbon alone or carbon and also Cr or Ti.

Summarizing the advantageous effects of the invention, according to the piston ring of the present invention, by making the carbon-based coating a multilayer coating of two types of layers which have a hardness difference of 500 to 1700 HV, increasing the thickness becomes possible and both a low friction property and excellent wear resistance are given. If the hardness difference is less than 500 HV, coating cracks cause the sliding surface to become rough, the coefficient of friction to become larger, and the wear of the opposing material to become greater as well. If the hardness difference becomes larger than 1700 HV, sliding causes the low hardness layer to become worn, stress concentrates at the surface relief parts of the high hardness layer (see FIG. 1B), the coating chips, the coefficient of friction becomes larger, and wear at the opposing material also becomes greater. If the overall thickness of the coating is less than 5.0 μm, the wear resistance becomes insufficient.

BRIEF DESCRIPTION OF THE DRAWINGS

These and other objects and features of the present invention will become clearer from the following description of the preferred embodiments given with reference to the attached drawings, wherein

FIGS. 1A to 1C show an embodiment of the present invention, wherein

FIG. 1A is a longitudinal cross-sectional view which shows one part of a piston ring,

FIG. 1B is an enlarged cross-sectional view of a coating part, and

FIG. 1C is a view which shows one part of a coating surface after sliding, and

FIG. 2 is a schematic view of a reciprocating wear tester.

DESCRIPTION OF PREFERRED EMBODIMENTS

Below, a preferred embodiment of the present invention will be explained while referring to the drawings.

A piston ring 1 has a carbon-based coating 2 formed on its outer circumference. The carbon-based coating 2 is a DLC (diamond-like carbon) coating comprised of a multilayer coating of two types of layers A and B with different hardnesses laminated in at least two layers each. In FIGS. 1A to 1C, the two types of layers A and B are laminated in six layers each. The surface-most layer is a layer A with a high hardness. The hardness difference between the two types of layers A and B is 500 to 1700 HV. The layer A with the high hardness (hereinafter referred to as the “high hardness layer A”) has a thickness of 5 to 90 nm, while the layer B with a low hardness (hereinafter referred to as the “low hardness layer B”) has the same or greater thickness. The overall thickness of the carbon-based coating 2 is 5.0 μm or more. The base material roughness of the outer circumference is 1.0 μmRz or less. The low hardness layer B may contain Cr or Ti. If Cr or Ti is contained, the adhesion with the high hardness layer A is improved more.

If the thickness of the high hardness layer A is smaller than 5 nm, the required widths of the high hardness layer A and the low hardness layer B which are exposed at the outer circumference after sliding (see FIG. 1C) cannot be sufficiently secured, the oil retention property falls, and the low friction effect becomes insufficient. If the thickness becomes larger than 90 nm, the widths of the high hardness layer A and the low hardness layer B which are exposed at the outer circumference after sliding become too great, coating cracks occur, the coefficient of friction becomes greater, and the wear of the opposing material also becomes greater. If the base material roughness of the outer circumference is larger than 1.0 μmRz, sliding causes the low hardness layer B to wear, stress to concentrate at surface relief parts of the high hardness layer A (see FIG. 1B), the coating to chip, the coefficient of friction to become greater, and the wear of the opposing material to also become greater. If the surface-most layer is a high hardness layer A, the surface of the opposing material can be quickly smoothened.

The above carbon-based coating 2 is formed using the PVD method by alternate sputtering (forming low hardness layer B) and ion plating (forming high hardness layer A). The thicknesses of the layers are adjusted by changing the formation times, while the hardnesses are adjusted by changing the bias voltage.

Below, the results of use of a reciprocating wear tester to measure the self wear amount, opposing wear amount, and coefficient of friction will be explained.

FIG. 2 shows an outline of a reciprocating wear tester which is used for the test. A pin-shaped top test piece 10 is held by a holding block 11. From above, a hydraulic cylinder 12 is used to apply a downward load whereby a bottom test piece 13 is pressed against. On the other hand, a flat disk shaped bottom test piece 13 is held by a movable block 14 and is made to move back and forth by a crank mechanism 15. Reference numeral 16 indicates a load cell.

The test conditions were as follow:

-   Load: 100 N -   Speed: 600 cpm/min -   Lubrication oil: 5 W-20 -   Time: 1 hour

Top test piece: pin-shaped test piece for piston ring given DLC coating

-   Bottom test piece: flake shaped graphite cast iron material for     cylinder liner

TABLE 1 Elements A, B layer of B Base A layer B layer hardness layer material A layer hardness hardness difference other roughness thickness Self Opposing Coefficient HV HV HV than C μmRz nm wear μm wear μm of friction Ex. 1 2000 300 1700 — 0.2 5 0.5 1 0.04 Ex. 2 1900 1400 500 — 0.3 20 0.5 1 0.04 Ex. 3 2700 1000 1700 — 0.2 30 0.5 1 0.04 Ex. 4 1700 400 1300 — 0.2 90 0.5 1 0.04 Ex. 5 2500 1000 1500 Cr 0.3 80 0.5 1 0.04 Ex. 6 1700 400 1300 — 0.5 10 0.5 1 0.04 Ex. 7 1700 400 1300 — 1.0 10 0.5 1 0.04 Comp. Ex. 1 1800 1400 400 — 0.2 90 Coating 5 0.09 breaks Comp. Ex. 2 2800 1000 1800 — 0.3 90 2.0 4 0.07 Comp. Ex. 3 1900 1400 500 — 0.3 3 0.5 1 0.06 Comp. Ex. 4 1700 300 1400 — 0.2 110 2.0 5 0.09 Comp. Ex. 5 2000 300 1700 — 0.2 3 0.5 1 0.07 Comp. Ex. 6 1700 400 1300 — 1.2 10 1.5 2 0.07

While the invention has been described with reference to specific embodiments chosen for purpose of illustration, it should be apparent that numerous modifications could be made thereto by those skilled in the art without departing from the basic concept and scope of the invention. 

1. A piston ring which has a carbon-based coating formed over its sliding surface, in which piston ring, said coating is a multilayer coating comprised of two types of layers having different hardnesses laminated in at least two layers, a hardness difference between said two types of layers is 500 to 1700 HV, a high hardness layer has the same or greater thickness than a low hardness layer, and the coating as a whole has a thickness of 5.0 μm or more.
 2. A piston ring as set forth in claim 1, wherein said high hardness layer has a thickness of 5 to 90 nm.
 3. A piston ring as set forth in claim 1, wherein the surface on which said coating is formed has a base material roughness of 1.0 μmRz or less.
 4. A piston ring as set forth in claim 1, wherein a surface-most layer of said coating is a high hardness layer.
 5. A piston ring as set forth in claim 1, wherein a low hardness layer of said coating contains carbon alone or carbon and also Cr or Ti. 